540  
FXUS02 KWBC 030714  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
213 AM EST FRI DEC 03 2021  
 
VALID 12Z MON DEC 06 2021 - 12Z FRI DEC 10 2021  
   
..OVERVIEW
 
 
THE MEDIUM RANGE PERIOD (MONDAY TO FRIDAY NEXT WEEK) WILL BE  
CHARACTERIZED BY A BROAD MEAN TROUGH ACROSS MUCH OF THE LOWER 48,  
WITH PERIODS OF SHORTWAVE ENERGY MOVING THROUGH THE TROUGH PUSHING  
LOW PRESSURE/FRONTAL SYSTEMS ACROSS THE COUNTRY. ROUNDS OF  
PRECIPITATION ARE FORECAST FOR THE WEST, WHILE RAIN IS LIKELY TO  
SPREAD INTO THE SOUTHEASTERN QUADRANT OF THE COUNTRY WITH PERHAPS  
SOME SNOW IN NORTHERN AREAS, BUT THERE IS STILL A GOOD BIT OF  
UNCERTAINTY WITH SPECIFIC WEATHER IMPACTS.  
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
THE 12/18Z (YESTERDAY) MODEL CYCLE REMAINS IN FAIRLY GOOD  
AGREEMENT WITH THE LARGE-SCALE PATTERN, WHILE ASPECTS OF THE  
SMALLER-SCALE DETAILS REMAIN QUITE VARIABLE. AT THE BEGINNING OF  
THE PERIOD MONDAY, THESE DIFFERENCES INCLUDE A SLOWER UPPER TROUGH  
OVER THE MIDWEST IN THE 12Z UKMET COMPARED TO THE SLIGHTLY FASTER  
CONSENSUS OF OTHER GUIDANCE, BUT OTHER THAN THE UKMET THERE WAS  
REASONABLY AGREEABLE TIMING WITH A GREAT LAKES SURFACE LOW AND THE  
TIMING OF ITS TRAILING COLD FRONT, MAYBE JUST SLIGHTLY FASTER THAN  
THE PREVIOUS FORECAST. ANOTHER DIFFERENCE AMONG GUIDANCE MONDAY  
WAS THAT THE GFS RUNS (AND MANY GEFS ENSEMBLE MEMBERS) CONTINUE TO  
BE THE STRONGEST WITH A POTENT SHORTWAVE ENTERING THE PACIFIC  
NORTHWEST, WHICH AS IT DIVES SOUTH INTO THE INTERMOUNTAIN WEST BY  
TUESDAY HAS IMPLICATIONS FOR ITS SPEED AND POSSIBLE FLOW  
SEPARATION. HOWEVER, AT THE SURFACE THE LOW PRESSURE SYSTEM IS  
WELL CLUSTERED OFFSHORE OF THE PACIFIC NORTHWEST ON MONDAY.  
MEANWHILE, SOUTHERN STREAM ENERGY NEAR BAJA CALIFORNIA MONDAY HAS  
SHOWN A FASTER TREND MOVING EASTWARD AND GETTING ABSORBED BY  
TUESDAY COMPARED TO SLOWER PREVIOUS GUIDANCE, WHICH IS REFLECTED  
IN THE WPC FORECAST.  
 
BY WEDNESDAY, RECENT GUIDANCE GENERALLY SHOWS A HINT OF A POSITIVE  
TILT TO THE MEAN TROUGH, WITH THE EXCEPTION OF THE 12Z CMC WITH  
ITS DIFFERENT PHASING OF SHORTWAVE ENERGY THAT IS NOT REFLECTED IN  
ITS NEW 00Z RUN EITHER, SO LEANED AWAY FROM THAT SOLUTION. A  
NOTABLE MODEL DIFFERENCE BY THURSDAY-FRIDAY IS THE DEGREE TO WHICH  
ENERGY DROPS SOUTHWARD THROUGH THE WEST AS WELL AS ITS TIMING. THE  
GFS AND CMC RUNS SHOW STRONGER ENERGY CREATING A DEEPER WESTERN  
TROUGH COMPARED TO THE ECMWF, WHICH SHOWS A COUPLE ROUNDS OF  
WEAKER SHORTWAVE ENERGY PASSING THROUGH THE MEAN TROUGH.  
CONFIDENCE IS LOW WITH THIS, GIVEN UNCERTAINTY WITH THE SHORTWAVE  
ENERGY STEMMING FROM HIGH LATITUDES, AND SPAGHETTI PLOTS OF 500 MB  
HEIGHTS SHOW THE OPERATIONAL GFS AND CMC ARE WITHIN IN THE  
GUIDANCE ENVELOPE BUT ARE ON THE STRONGER END OF SOLUTIONS WITH  
TROUGHING IN THE WEST. THEN IN THE EAST, THERE IS UNCERTAINTY WITH  
A SURFACE LOW SPINNING UP ALONG THE COASTAL NORTHEAST LATE  
WEDNESDAY INTO THURSDAY, WITH NO GOOD CONSENSUS POSITION YET, AND  
THIS WILL HAVE IMPLICATIONS FOR PRECIPITATION ALONG THE EASTERN  
SEABOARD.  
 
THUS THE WPC FORECAST BEGAN THE PERIOD USING A BLEND OF THE  
12Z/18Z GFS AND 12Z ECMWF AND CMC, PHASING OUT THE CMC FROM  
MIDWEEK ONWARD AND INCREASING PROPORTIONS OF THE AGREEABLE GEFS  
AND EC ENSEMBLE MEANS (TO 60% BY DAY 7/FRIDAY). THIS SERVED TO  
KEEP PROPER MAGNITUDE OF FEATURES WHEN THERE IS GOOD MODEL  
CONSENSUS, AND LESSENED THE EMPHASIS ON INDIVIDUAL MODELS BY THE  
LATTER PART OF THE PERIOD AS UNCERTAINTY INCREASES.  
   
..WEATHER/HAZARDS HIGHLIGHTS
 
 
AS A RELATIVELY STRONG SURFACE LOW PRESSURE SYSTEM IS POSITIONED  
ACROSS THE GREAT LAKES EARLY MONDAY, SNOW IS EXPECTED THERE WHILE  
STRONG WINDS ARE POSSIBLE IN THE TIGHT PRESSURE GRADIENT BEHIND  
THE LOW. CONDITIONS WILL BE FAVORABLE FOR LAKE EFFECT SNOW BEHIND  
THE SYSTEM THROUGH TUESDAY. ADDITIONALLY, RAIN IS FORECAST ALONG  
THE TRAILING COLD FRONT ASSOCIATED WITH THE LOW FOR THE OHIO  
VALLEY TO SOUTH-CENTRAL U.S., BUT AMOUNTS ARE LIKELY TO BE  
TAPERING DOWN BY MONDAY. HOWEVER, GULF MOISTURE SHOULD INCREASE  
AGAIN BY TUESDAY INTO WEDNESDAY AND CAUSE MODERATE TO POSSIBLY  
LOCALLY HEAVY RAINFALL FROM THE LOWER MISSISSIPPI VALLEY INTO THE  
TENNESSEE VALLEY AND SOUTHEAST. MOISTURE SPREADING FARTHER NORTH  
INTO COOLER AIR COULD LEAD TO SNOW ACROSS THE NORTHERN LATITUDES,  
WITH RELATIVELY HIGHER PROBABILITIES FROM THE GREAT LAKES INTO  
NORTHERN NEW ENGLAND.  
 
A COUPLE OF SHORTWAVES MOVING ACROSS THE WEST WILL CAUSE  
INCREASING PRECIPITATION CHANCES THERE, WITH FAVORED WINDWARD  
TERRAIN SEEING THE HIGHEST AMOUNTS. THE PACIFIC NORTHWEST INTO  
MUCH OF THE INTERMOUNTAIN WEST AND ROCKIES SHOULD SEE  
PRECIPITATION MONDAY AND CLEARING OUT BY TUESDAY WHILE CHANCES  
RAMP UP IN THE SOUTHWEST. THEN THE NEXT ROUND IS LIKELY FOR THE  
NORTHWEST ON WEDNESDAY AND SPREADS INTO CALIFORNIA EASTWARD BY  
THURSDAY AS ENERGY ALOFT DROPS SOUTHWARD. GENERALLY HIGHER  
ELEVATION SNOW AND LOWER ELEVATION RAIN IS FORECAST, THOUGH  
VARYING THROUGHOUT THE PRECIPITATION EVENTS AND PERHAPS RELATIVELY  
LOW SNOW LEVELS ON AVERAGE GIVEN PERIODS OF BELOW NORMAL HEIGHTS  
ALOFT.  
 
CHILLY TEMPERATURES ARE LIKELY ACROSS THE FAR NORTH-CENTRAL U.S.  
ON MONDAY AND TUESDAY GIVEN SNOW COVER FROM SUNDAY'S SYSTEM, WITH  
TEMPERATURES 10-20F BELOW NORMAL. MEANWHILE THE EASTERN SEABOARD  
CAN EXPECT TEMPERATURES ABOUT 10-20F ABOVE NORMAL ON MONDAY AHEAD  
OF A COLD FRONT BRINGING TEMPERATURES NEAR AVERAGE BY TUESDAY.  
OVERALL TEMPERATURES SHOULD BE WITHIN ABOUT 10 DEGREES OF NORMAL  
IN MOST PLACES TUESDAY AND WEDNESDAY, BEFORE A WARMUP BY THURSDAY  
AND FRIDAY FROM THE PLAINS EASTWARD, WITH 10-15F ABOVE AVERAGE  
TEMPERATURES. A COOLDOWN IS POSSIBLE IN THE NORTHWEST BY THE  
LATTER PART OF THE WEEK WITH THE POTENTIAL FOR TROUGHING ALOFT  
THERE.  
 
TATE  
 
ADDITIONAL 3-7 DAY HAZARD INFORMATION CAN BE FOUND ON THE WPC  
MEDIUM RANGE HAZARDS OUTLOOK CHART AT:  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP  
 
WPC MEDIUM RANGE 500MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDICES ARE AT:  
 
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
 

 
 

 
 
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